1. Technical Field
The present invention relates to a power conversion apparatus having a function of protecting a motor drive inverter from an overcurrent.
2. Background Art
Generally, an overcurrent protection function is provided in an inverter for the purpose of protecting the inverter from an overcurrent. Although an overcurrent detection level in this case can be determined in accordance with an allowable current of semiconductor switching devices such as IGBT's (Insulated Gate Bipolar Transistors) which are constituent parts of the inverter, the overcurrent detection level is generally fixed in an inverter for a variable speed use.
Almost all vehicle drive motors in hybrid automobiles or electric automobiles are controlled in such a manner that a constant torque region in which the torque is constant is provided when the number of revolutions is less than a certain revolution number n1 (e.g. n1=1500 [r/min.]), and that a constant output region in which the output is constant is provided when the number of revolutions is equal to or larger than the revolution number n1, as shown in FIG. 7. When a permanent magnet synchronous motor such as an interior permanent magnet synchronous motor is used as this type motor, the current is not completely proportional to the torque but decreases as the necessary torque decreases. That is, according to the characteristic of FIG. 7, the current decreases as the number of revolutions increases.
Consider now the case where a state in which the current of the motor reaches 300[%] of the rated current is detected as an overcurrent. If a permanent magnet synchronous motor is used, the no-load induced voltage in the case where the number of revolutions is 6000 [r/min.] is four times as large as that in the case where the number of revolutions is 1500 [r/min.]. That is, when the inverter is stopped in an emergency during overcurrent detection, regenerative energy becomes larger as the motor rotates at a higher speed. When the inverter is stopped in an emergency in this manner during high-speed rotation of the motor, large energy is regenerated to a DC-side capacitor of the inverter. As a result, there is a possibility that the capacitor will be broken down by an overvoltage applied on the capacitor.
Here, when all gates of the inverter are shut off to stop the inverter in an emergency while the motor rotates, the state in which the voltage of the capacitor increases is confirmed by a simulation and a result thereof will be therefore described.
FIG. 8 is a circuit diagram of a power conversion apparatus used in the simulation. In FIG. 8, the reference sign 10 designates a DC power supply; 20, a power supply cut-off switch; 30, a capacitor; 40, a three-phase voltage type inverter having semiconductor switching devices bridge-connected; 60, a controller; 61, a current detecting unit; 65, a gate signal generating unit; M, a three-phase motor driven by the inverter 40; P and N, DC input terminals of the inverter 40; and U, V and W, AC output terminals.
In FIG. 8, the power supply cut-off switch 20 is opened in the condition that the motor M is driven by the inverter 40 and, at the same time, all the semiconductor switching devices of the inverter 40 are turned off (all the gates are shut off) by the gate signal generating unit 65. Incidentally, the rated output of the motor M is 20 [kW], the no-load induced voltage at the revolution speed of 8000 [r/min.] is 519 [V], the rated current is 60 [A], the DC intermediate voltage (the voltage of the capacitor 30) is 650 [V], and the capacitance value of the capacitor 30 is 400 [μF].
FIG. 9 is a simulation result when all the gates are shut off while the motor M is operated at 40 [kW] which is twice as large as the rated output. The current of the motor M is 49 [A] (about 82[%] of the rated current). It is found from FIG. 9 that the voltage of the capacitor 30 increases to about 812 [V] because all the gates are shut off.
Next, FIG. 10 is a simulation result when all the gates are shut off while the motor M is operated at 120 [A] which is twice as large as the rated current. It is found from FIG. 10 that the voltage of the capacitor 30 increases to about 961 [V] because all the gates are shut off.
As described above, when all the gates of the inverter 40 are shut off while the motor M rotates at a high speed, a high voltage is applied on the capacitor 30. Accordingly, assuming that, for example, the withstand voltage of the capacitor 30 is 900 [V], then there is a possibility that the capacitor 30 will be broken down because a voltage not lower than the withstand voltage is applied on the capacitor 30 when all the gates are shut off in the case as shown in FIG. 10.
Some inverters have a current limiting function in addition to the overcurrent protection function. Here, the current limiting function is a function by which the current is suppressed so that a current equal to or larger than a preset current limit level does not flow while operation of the inverter is continued.
As a background-art technique provided with the current limiting function, there is known a power conversion apparatus described in PTL 1 (identified further on). The power conversion apparatus has a function of adjusting a current limit level so that an inverter is not stopped due to an overcurrent when failure such as momentary interruption of an AC power supply (hereinafter referred to as “momentary interruption” simply) occurs and power is then recovered to restart a motor.
FIG. 11 shows the background-art technique according to PTL 1. The reference sign 100 designates an AC power supply; 200, a main circuit part of a power conversion apparatus; 201, a rectifier circuit; 202, a capacitor; 203, an inverter circuit; 301, a current detector; 302, a current limit level calculator; 303, a comparison device; 304, a phase detector; and 305 and 306, gate controllers.
Operation of FIG. 11 will be described briefly as follows. When, for example, failure such as momentary interruption of the AC power supply 100 occurs, the gate controller 305 stops the rectifier circuit 201 and the gate controller 306 stops the inverter circuit 203. In addition, the gate controller 306 calculates a frequency command of the inverter circuit 203 for restarting, based on the phase of the motor M detected by the phase detector 304. When the power of the AC power supply 100 is then recovered, the gate controllers 305 and 306 restart the rectifier circuit 201 and the inverter circuit 203.
On this occasion, the comparison device 303 compares an output current of the inverter circuit 203 with a current limit level calculated from a predetermined function by the current limit level calculator 302 in accordance with each phase based on a speed command. The gate controller 306 gate-blocks only the phase which has reached the current limit level for a predetermined period to thereby prevent the output current of the inverter circuit 203 at the phase from becoming excessive.
In this manner, even when the current at a certain phase reaches the current limit level, the power conversion apparatus can be operated continuously.